In 2018, seal attacks resulted in the deaths of more than half a million farmed Scottish salmon, costing farmers in excess of £14 million. The fish died as a result of either physical attack – the seals target the salmons’ livers, taking a single fatal bite, or the stress the predator attacks caused.
Acoustic deterrent devices (ADDs) have been used to try and dissuade seals from attacking fish farms or entering river systems for the last 40 years. They are one of number of predator management systems, including reinforced or double nets, seal blinds and new pen installation techniques, invested in by Scottish fish farmers to protect both their livestock and the predators.
The first systems were underwater speakers that played recordings of orcas (killer whales), the only natural predator of seals, but the seals quickly learned that the threat was not real. Most modern ADDs use underwater transducers which convert electrical energy into sound waves that can be used in two ways. Firstly, psychological systems which use random frequency sweeps and tones to try and unsettle an approaching animal. Typically these work very well for short periods of time but the sound programme must be changed regularly to remain an effective deterrent. Secondly, physiological systems use sound pressure at a specific frequency to cause approaching seals discomfort at close range. The sound wave is usually just white noise (a series of clicks) and unless the sound pressure is high enough the seal (and other marine mammals) will not be affected.
By using a number of strategically placed ADDs, a fish farm can create an invisible, sonic fence around all of its pens. While seals and cetaceans swimming nearby can hear the ADDs it’s not until they get very close to them that it appears to become a dissuading irritant. Scottish fish farmers regularly log sightings of seals, dolphins, harbour porpoises and even orcas swimming in the immediate vicinity of farms with ADDs in active operation.